The present invention relates to polarization conversion systems and, in particular, to an integrating polarization conversion system that maximizes illumination brightness.
Various display devices, including electronic projectors (e.g., liquid crystal display projectors), employ illumination sources to illuminate the display device. An important performance requirement of such display devices is that the illumination light be uniform over the entire area of the display device. Variations in illumination uniformity are readily discernible and are an undesirable artifact. In addition, some display devices, including many that employ liquid crystal cells to impart image information, require that the illumination light be polarized for the display device to be operable.
U.S. Pat. No. 5,978,136 describes a polarization illumination device directed to providing polarized illumination light with improved illumination uniformity. This polarization illumination device includes a pair of lens arrays and an optical element containing two polarization beamsplitter arrays for converting polarized light with random polarization directions to one type of polarized light. Each polarization beamsplitter array includes multiple polarization beamsplitters with inclined surfaces that are parallel with each other.
The inclined surfaces of the two polarization beamsplitter arrays face each other generally to form a chevron configuration. To accommodate the center of the chevron, the two polarization beamsplitter arrays are spaced apart from each other to form a central stripe region. Due to the absence of a polarization beamsplitter, illumination light in the central stripe region is not polarized and, as a consequence, is not utilized for display illumination.
In accordance with the present invention, a polarization conversion system provides generally uniform polarized illumination light while maximizing illumination brightness by utilizing all illumination light, including the typically brightest illumination light available along a central region. In one implementation, the polarization conversion system includes a pair of lens arrays that successively receive light from an illumination source. Each lens array has multiple lenslets, and corresponding lenslets in the two arrays are optically aligned with each other.
A planar array of polarization beamsplitters is positioned adjacent the latter lens array. Each polarization beamsplitter includes a pair of elongated right-angle prisms having their respective inclined faces positioned against each other and their lengths extending vertically across multiple lenslets of the latter lens array. The polarization beamsplitter array includes coplanar top and bottom array segments, the inclined faces of the prisms of the polarization beamsplitters of the top array segment being oriented at substantially one angle (e.g., +45xc2x0) and the inclined faces of the prisms of the polarization beamsplitters of the bottom array segment being oriented at substantially a reversed angle (e.g., xe2x88x9245xc2x0). The top and bottom polarization beamsplitter array segments meet each other end-to-end with substantially no space between them. As a result, the polarization conversion system of the present invention provides generally uniform polarized illumination light while maximizing illumination brightness.
As another implementation the present invention, each of the lens arrays includes coplanar top and bottom lens array segments that each include plural lenslets. The lenslets of the top and bottom lens array segments are aligned with the polarization beamsplitter arrays in the top and bottom polarization beamsplitter array segments. In addition, the lenslets and polarization beamsplitters of the top segments are laterally offset relative to the lenslets and polarization beamsplitters of the bottom segments. This lateral offset provides increased uniformity of illumination and alleviates shadowing due to the x-cube centerline discontinuity. The horizontal frequency content of the angular distribution of the illumination is increased, basically doubled, with reduced amplitude, and further approximates the smooth xe2x80x98Gaussianxe2x80x99 distribution that is expected by the optical system, the projection lens and the human eye. In contrast, a conventional illumination system provides hot pencils of rays that cause discontinuous artifacts, sharp-edged shadows, etc.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.